The invention is concerned with the manufacture of mineral fibres of the type comprising glass fibres of fine diameter, especially of at most 3 xcexcm, for the purpose of incorporating them into papers used more particularly for producing aerosol filters or battery separators.
Specifically, the patents EP-0,267,092 and EP-0,430,770 disclose fibre-containing papermaking sheets for such uses. These fibres are usually a mixture of too types of fibre: xe2x80x9cfinexe2x80x9d fibres having a mean diameter of the order of at least 2 to 3 xcexcm, and xe2x80x9cvery finexe2x80x9d fibres having a diameter lower than 1 xcexcm. The first are intended, more particularly, for reinforcing the paper mechanically and giving it the necessary xe2x80x9cbulkxe2x80x9d, whilst the second provide the porosity which gives it its filtration properties. As proposed in the two patents mentioned above, these two types of fibre may advantageously be obtained by means of a method called internal centrifuging which is widely used, moreover, for manufacturing fibres used to produced thermal or acoustic insulation products. In outline, this method involves introducing a stream of melted glass into a centrifuge, also called a fibre-forming disk, rotating at high speed and perforated on its periphery with a very large number of orifices, through which the glass is discharged in the form of filaments under the effect of the centrifugal force. These filaments are then subjected to the action of an annular drawing flow of high temperature and velocity which runs along the wall of the centrifuge and which thins them and converts them into fibres. The fibres formed are carried by this gaseous drawing flow towards a receiving device generally consisting of a gas-permeable band. This known method has undergone many improvements, including especially those taught in the patents BP-B-0,189,534 or EP-B-0,519,797.
For thermal or acoustic insulation, the fibres manufactured by means of such a method generally have a diameter greater than 3 xcexcm, usually approximately 4 to 4.5 xcexcm up to 12 xcexcm: the method therefore cannot be used in its conventional operating parameters in order to obtain the xe2x80x9cfinexe2x80x9d or xe2x80x9cvery finexe2x80x9d fibres referred to above. Some adaptations are therefore necessary in order to obtain fine fibres by means of this method. Thus, the above mentioned patent EP-0,267,092 proposes, specifically, a particular selection of the velocity of the drawing gases in order to obtain the said fibres.
The object of the invention, then, is an improvement in the device and method for the formation of fibres by the internal centrifuging of fine mineral fibres of a diameter of at most 3 xcexcm, the said improvement being aimed especially at the quality of the fibres obtained and/or the production yield of these.
The subject of the invention is, first of all, a device for the internal centrifuging of mineral fibres of a diameter below or equal to 3 xcexcm, comprising a centrifuge equipped with a peripheral band perforated with orifices and which has a fibre-forming height below or equal to 35 mm, preferably below or equal to 32.5 or 30 mm, and, for example, between 16 and 32.5 mm.
Within the scope of the invention, the xe2x80x9cfibre-forming heightxe2x80x9d of the centrifuge is defined by the distance separating the highest point of the peripheral band from the xe2x80x9clowestxe2x80x9d orifices of the latter, xe2x80x9chighxe2x80x9d and xe2x80x9clowxe2x80x9d being understood with reference to the centrifuge arranged in the centrifuging position, that is to say along a substantially vertical axis (of rotation).
Conventionally, fibres of standard diameter for insulation are manufactured with the aid of centrifuges with a wide peripheral band having, to give an order of magnitude, a fibre-forming height of at least 40 mm. It is, in fact, expedient to select a considerable fibre-forming height, since this makes it possible to increase the number of orifices in the peripheral band and to have a large number of rows of orifices leading to an increase in the production capacity, often expressed by the xe2x80x9crun-offxe2x80x9d equal to the number of kg of glass fibres manufactured per day and per centrifuge. This reasoning has its limits, of course, since the production capacity is also linked to many other parameters and must not be obtained at the expense of too significant a quantity of unformed fibres, grains, risks of malfunctioning of the centrifuge, etc. Thus, it is also necessary, for example, to take into account the temperature gradients which may be established over the height of the peripheral band or the fact that the high-temperature drawing gases have or do not have the same impact on all the rows of orifices.
It became apparent that the quality of the so-called xe2x80x9cfinexe2x80x9d fibres, with a diameter of less than 3 xcexcm, could be increased considerably by selecting a particular fibre-forming height below 35 to 30 mm or less, that is to say at values well below those used conventionally and those used hitherto to make so-called xe2x80x9cfinexe2x80x9d fibres of less than 3 xcexcm. With such a type of centrifuge of lower height, it was possible to see that the xe2x80x9cfinexe2x80x9d fibres produced were much more stable dimensionally, with a much smaller quantity of defects of the type comprising unformed fibres/grains or of the type comprising amalgams of re-agglutinated fibres, and that they had highly satisfactory mechanical properties, especially in terms of yield strength under traction.
Moreover, the use of the centrifuge was simpler and less complicated than that encountered hitherto in the production of xe2x80x9cfinexe2x80x9d fibres. Furthermore, this increase in quality and this higher industrial feasibility were not obtained at the expense, of an economically unacceptable reduction in the production capacity, on the one hand because xe2x80x9cfinexe2x80x9d fibres of very particular use have a high added value and because it is also possible to have a production capacity a little lower than that obtained in the case of standard fibres, and, on the other hand, because it has been possible to limit at least partially the fall in production capacity by adjusting, especially increasing, the perforation density of the peripheral band.
Advantageously, the centrifuge according to the invention is selected with a mean diameter below or equal to 800 mm, especially of at least 200 mm, for example of approximately 200, 400 or 600 mm.
In order to obtain xe2x80x9cfinexe2x80x9d fibres with a diameter of at most 3 xcexcm, it is preferable for the orifices of the centrifuge to have a diameter of at most 1.5 mm, for example at most 1.2 mm, especially in between 1.1 and 0.5 mm, for example between 0.9 and 0.7 mm (reference is made, here, to a xe2x80x9cdiameterxe2x80x9d, since these orifices are usually all selected circular, but it is not ruled out that the orifices are not circular, in which case xe2x80x9cdiameterxe2x80x9d must be understood as meaning xe2x80x9clargest dimensionxe2x80x9d).
According to a preferred embodiment, the orifices of the peripheral band of the centrifuge are grouped in rows, conventionally rows arranged in concentric circles over the height of the band. In the invention, then, it is advantageous if at least two adjacent rows have orifices of different diameters, and, more precisely, if the rows, have decreasing orifice diameters from the top of the peripheral band downwards (usually, all the orifices of the same row have the same diameter). It is thus possible to provide, from the top downwards, n rows of orifices of a given diameter, then p rows of orifices of a smaller diameter, then t rows of orifices of an even smaller diameter, etc. With: n, p and txe2x89xa71.
An improvement in the fibre-forming quality was found when a kind of xe2x80x9cgradientxe2x80x9d decreasing from the top downwards in the size of orifices was established in this way. It was thus possible to reduce the differences in the way in which the filaments emanating from the highest rows were formed into fibres from those of the lowest rows; this xe2x80x9cgradientxe2x80x9d allows a development of the primary filaments at the outlet of the orifices and drawing which limits the path intersections, and therefore the impacts, between the fibres being drawn, which come from different rows of orifices, hence the increase in quality which is observed.
Preferably, the orifices of the centrifuge are grouped in rows spaced from one another at a distance of between 1 and 2 mm, especially between 1.2 and 1.8 mm, preferably with a pitch of offset from one row to another of between 1 and 2 mm, especially between 1.8 and 1.3 mm, and a spacing between two orifices of the same row of between 2.2 and 1.4 mm, especially between 2.1 and 1.6 mm.
Advantageously, the device according to the invention comprises a high-temperature gaseous drawing means in the form of an annular burner. It has thus proved particularly expedient, in controlling the production of xe2x80x9cfinexe2x80x9d fibres, to provide a means for channelling these high-temperature drawing gases and/or the mineral material ejected from the centrifuge and being converted from filaments into fibres, this means being in the form of a ring blowing non-hot gas, for example at ambient temperature. There may also be optionally be provided a means for the external heating of the walls of the centrifuge, especially in its lowest part, in the form of an annular inductor. All these elements are particularly described, in terms of their operating principle, in the above mentioned patents EP-0,189,354 and EP-0,519,797.
A heating means xe2x80x9cwithinxe2x80x9d the centrifuge, of the internal burner type, may also be used. This may perform various functions, especially that of terminating the thermal conditioning of the melted glass in the xe2x80x9cbasketxe2x80x9d of the centrifuge (term explained below with the aid of the figures), of maintaining the reserve of glass in the centrifuge at a suitable temperature, and, finally, of continuously remelting the fibres liable to have been re-agglutinated on the outer walls of the centrifuge.
The xe2x80x9cexternalxe2x80x9d beating means of the annular-inductor type may advantageously be combined with this internal hem ring means, and it also makes it possible to control more effectively the temperature of the reserve of glass and the remelting of the re-agglutinated fibres. Specifically, it was found that, in general, for low run-offs, it was sufficient simply to employ an internal burner, whereas, for high run-offs, the annular inductor proved necessary, the internal burner optionally completing the latter in an advantageous way.
The subject of the invention is also a method for the formation of xe2x80x9cfinexe2x80x9d mineral fibres having a diameter below or equal to 3 xcexcm by internal centrifuging associated with high-temperature gaseous drawing, especially using the device described above. According to this method, the material to be formed into fibres is poured into a centrifuge, the peripheral band of which is perforated with orifices, with a fibre-forming height (defined above) of the centrifuge of below or equal to 35 mm, especially of at most 32.5 mm, and advantageously of between 16 and 32.5 mm.
The centrifuge is advantageously that having the characteristics described above.
Hot gaseous drawing is advantageously carried out by means of an annular burner, the operating parameters of which may be selected in the following way:
the temperature of the gases at the burner outlet may preferably be not at at least 1350xc2x0 C. especially at least 1400xc2x0 C., and, for example, between 1400 and 1500xc2x0 C., especially between 1430 and 1470xc2x0 C. The temperature is subsequently adjusted as a function of the type of mineral fibre composition, depending above all on its viscosimetric behaviour. These temperature values have proved the most favourable for producing xe2x80x9cfinexe2x80x9d fibres,
advantageously, the velocity of the gases emitted by the burner is also set at at least 200 m/s, (measured just at the outlet of the lips of the burner), especially at values of between 200 and 295 m/s,
finally, the annular width of the gases at the burner outlet is preferably set at values of between 5 mm and 9 mm.
When the method of the invention employs a means for channeling the hot drawing gases and/or the material ejected from the orifices of the centrifuge under the effect of the centrifugal force, this means is advantageously a ring blowing the gas which is at most at ambient temperature and under a supply gas pressure of between 0.5 and 2.5.105 Pa, especially 0.7 to 2.10xe2x88x925 Pa.
In fact, within the context of the invention, the use of a blowing ring has proved particularly advantageous: its jets of xe2x80x9ccoldxe2x80x9d air (usually at ambient temperature) have a cooling effect which is beneficial to the fibres being drawn, particularly where xe2x80x9cfinexe2x80x9d fibres relevant to the invention are concerned. Specifically, this cooling avoids xe2x80x9cover-drawingxe2x80x9d of the fibres and turbulence which tends to shorten them and embrittle than. It therefore contributes to ensuring that the fibres have good mechanical properties, this being all the more important since, here, the fibre-forming zone is of reduced height, as compared with fibre formation of standard dimension: thus, there is a clear, and advantageous, distinction between, on the one hand, the drawing zone subjected to the influence of the annular burner and the cooling zone subjected to the influence of the annular blower (the latter remains optional, however).
An inductor may be employed in order to heat the lowest zone of the centrifuge and avoid or limit the occurrence of a temperature gradient over the height of the centrifuge. However, since the centrifuge has a relatively low height here, the said inductor is merely optional. As mentioned above, an internal burner may also be employed cumulatively or alternatively.
The method according to the invention makes it possible to alternate production capacities perfectly compatible with the economic requirements, in view of the quality of the fibre obtained (for example, generally less than 1.5 to 2% by weight of grains/unformed fibres). It has thus been possible to obtain run-offs of fibres of a diameter of between 2 and 3 xcexcm, for example 2.6 xcexcm, of the order of 0.5 kg per day and per centrifuge orifice, thus corresponding to industrial run-offs of the order of 2500 to 7700 kg per day and per centrifuge for centrifuges of a mean diameter of 400 mm and a fibre-forming height of between 16 and 32.5 mm.
The subject of the invention is alto the use of the fibres obtained by means of the device and/or method described above, and having a diameter of at most 3 xcexcm, in the manufacture of paper, especially for the purpose of producing filters or battery separators.
The subject of the invention is also paper incorporating these xe2x80x9cfinexe2x80x9d fibres of a diameter of at most 3 xcexcm (usually between 2 and 3 xcexcm, and xe2x80x9cvery finexe2x80x9d mineral fibres of a diameter of at most 1 xcexcm (usually between 0.2 and 0.8 xcexcm). Although the invention tends to refer to xe2x80x9cfinexe2x80x9d fibres., it is not ruled out that it also relates to the manufacture of xe2x80x9cvery finexe2x80x9d fibres.
Advantageously, the mass ratio between the two types of fibre is between 25/75 and 75/25, especially in the neighbourhood of 60/40 to 40/60.